1. Field of the Invention
The present invention relates to an organic electroluminescent (EL) device, and more particularly, to an anode of an organic EL device, by which optical efficiency and stability of the organic EL device is enhanced.
2. Discussion of the Related Art
Generally, an organic electroluminescent (hereinafter abbreviated EL) device emits light in a manner that an electron and hole are recombined to be annihilated by injecting an electric charge into an organic layer between an electron injection electrode (cathode) and a hole injection electrode (anode).
The organic EL device is a next generation display device that can be driven by a low voltage with less power consumption.
The organic EL device can be categorized according to a light-emitting mechanism into a bottom-emission type of emitting light toward a substrate and a top-emission type of emitting light toward an opposite side of substrate.
The top-emission type organic EL device consists of an anode on a substrate to receive an injected hole therein and to play a role of a reflective layer of emitted light, an organic layer including at least one layer of organic matter, and a cathode receiving an injected electron to transmit the emitted light.
Among the elements of the organic EL device, the anode needs features of high work function, high reflectivity, low resistance, low surface illumination intensity, high adhesion to substrate, high etching feasibility, superior environmental characteristic, etc.
Al or Al-alloy is generally used in forming a reflective layer adopted by an optical record medium such as CD, DVD, and the lime or an optical reflective layer frequently adopted by a reflective type STN liquid crystal display device, organic EL device, and the like.
Al or Al-alloy layer has favorable reflectivity (80%˜90%) and low electric resistance. An intensified Al2O3 layer is formed on a surface of the Al or Al-alloy layer. Hence, the Al or Al-alloy layer has stable corrosion resistance at an ambience of air.
Accordingly, many efforts were made to apply the Al or Al-alloy to an anode of the top-emission type organic EL device. Yet, the work function of the organic EL device is low to reduce the I-V characteristic and optical efficiency.
In the top-emission type organic EL device, an anode material employs such a single element of high work function as Cr, Ni, Mo, Ag, and the like or an alloy containing at least two of the elements.
However, such a substance as Cr, Ni, Mo, and the like has high work function but shows low reflectivity. The Ag single element or its alloy has high work function and reflectivity but shows inferior environmental characteristic and poor corrosion resistance.